1. Field of the Invention
The present invention relates to a piston position detector for a fluid pressure cylinder acting by air pressure or oil pressure, wherein the position of a piston sliding within a cylinder is detected from the outside of the cylinder, and the piston is stopped by the detection signal and the operating signal is transmitted to the other actuator, and more particularly to a position detector wherein the cylinder is made of a non-magnetic material such as aluminium, and the position of the piston is detected based on the variation of the magnetic field formed by a permanent magnet mounted on the piston.
2. Description of the Prior Art
Such a piston position detector in the prior art is disclosed in the Japanese Patent Publication No. 47-46840 where a reed switch responding to only magnetic flux in the moving direction of piston is used as the detection element, and in the Japanese Utility Model Unexamind Publication No. 58-129106 where a plurality of magnetoresistive elements are assembled in a bridge and used as a detection element in order respond to the strength of an acting magnetic flux irrespective of its direction. Usually, a permanent magnet mounted on a piston is ring shaped and fitted to the outer circumfrence of the piston in a concentric relation, thereby the magnetic field formed by the permanent magnet becomes as shown in broken lines of FIG. 4(1). In this case, the X-axis is one passing through the center of the cross-section of the permanent magnet a in parallel to the axial center of the cylinder, that is, the moving axis of the permanent magnet a. Strength of the magnetic field on line b apart from the X-axis by distance y, irrespective of direction of the magnetic flux, is proportional to the total of the magnetic flux in all directions. As shown in broken lines of FIG. 4(2), the magnetic field strength on line b in FIG. 4(1), irrespective of direction, becomes strongest at the position corresponding to the center of the permanent magnet a and becomes weaker gradually at positions away from the center position towards both sides, thus distribution becomes mountain-like shaped. On the contrary, the magnetic field strength on line b in the direction of the line b becomes strongest at the position corresponding to the center of the permanent magnet a, and since the magnetic flux is orthogonal to the line b, at positions away from the center position towards both sides by distance x, the magnetic field strength on line b in the direction of the line b becomes zero once at the positions. At positions further away from the center, since the direction of the magnetic flux gradually becomes oblique, the component in the direction of the line b increases again so as to form a small mountain, and then the magnetic field strength in the direction of the line b decreases again with decrease of the flux density. Thus the magnetic field strength in the direction of the line b has such a distribution that the lower peaks are formed on both sides of the higher peak at the center.
Consequently, when the magnetic detection element responding to the strength of the magnetic flux, irrespective of its direction, is arranged on line b, since the magnetic detection element acts in a wide range of the stroke of the piston, detection of great accuracy cannot be effected. also when the magnetic detection element such as a reed switch responding only to magnetic flux in the moving direction of the piston is used, depending on the sensitivity setting, the detection signals are generated not only on the position corresponding to the center of the permanent magnet but also on positions slightly away from the center portion towards both sides, resulting in an erroneous operation.